Canker-resistant aspen tree 70-144-30-68-1

ABSTRACT

An aspen tree having rapid growth, good form, aesthetic appearance, and resistance to Hypoxylon canker caused by the fungus Hypoxylon mammatum.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support from a grant funded bythe United States Department of Agriculture (U.S.D.A.), U.S. ForestService, North Central Forest Experiment Station Cooperative AgreementNo. 13-80-23. The government may have certain rights in the invention.

This invention is a new and distinct variety of Aspen tree of thespecies known botanically as Populus tremuloides (Michx.) The varietyhas been identified by the number 70-144-30-68-1.

BACKGROUND INFORMATION

The genus Populus includes various species of trembling aspen,cottonwood, big tooth aspen, balm-of-gilead, white poplar and lombardypoplar. Many aspen trees are grown as forest trees, but aspen trees arealso frequently planted in windbreaks, parks, and home landscapes.

This variety of Populus tremuloides (Michx.) has been selected becauseof a combination of several desirable characteristics, including rapidgrowth, good tree form, aesthetic qualities, and resistance to Hypoxyloncanker caused by the fungus Hypoxylon mammatum.

All known varieties of aspen are quite susceptible to canker diseasecaused by infection with the fungus Hypoxylon mammatum. However, thecultivar described herein has exhibited good resistance to Hypoxylonmammatum, with calluses forming at the margins of cankers which slow orcompletely stop the canker growth.

This variety was selected as a ramet from a superior aspen clone growingin an aspen stand in Block 70 of the Pike Bay Experimental Forest on theChippewa National Forest, near Cass Lake, Cass County, Minn. Rootcuttings were taken in 1965 and individual trees were planted at theRosemount Agricultural Experiment Station, Rosemount, Minn. in 1968 andat the Oconto Seed Orchard on the Nicollet National Forest, Langlade,Wis. in 1971. The trees were planted at a spacing of 10×10 meters whichenhances canker infection and provides greater selection pressure toidentify resistant trees. Several times each growing season,observations have been made of these plantings of the variety withrespect to growth, form, resistance to disease and resistance toinsects.

The variety described herein has continued to exhibit a novelcombination of characteristics, including rapid growth, good form andaesthetic appearance, and most notably a good resistance to cankerdisease caused by infection from the fungus Hypoxylon mammatum.

Asexual Propagation

This variety has been asexually or vegetatively propagated from rootcuttings collected in the field, from single node explants in tissueculture, and from invitro-grown sterile root cultures. Plants have beengrown from all of such methods of propagation and have been planted inthe field at the Pike Bay Experimental Forest on the Chippewa NationalForest, near Cass Lake, Minn., at the Rosemount Agricultural ExperimentStation, Rosemount, Minn., and at the Oconto Seed Orchard on theNicollet National Forest, Langlade, Wis.

Asexual propagation of this variety may be easily accomplished by use ofroot suckers, under greenhouse conditions. It has also been determinedthat the tree of this variety may be vegatatively propagated by tissueculture techniques, whereas most other clones of aspen trees have notyet been successfully propagated by tissue culture.

It has been determined from observation of the numerous plantings,asexually propagated from this variety, that the characteristicsdescribed herein are firmly fixed through successive generations andthat the distinguishing characteristics hold true to form throughsucceeding propagations.

DESCRIPTION OF THE DRAWINGS

This new variety of aspen tree is illustrated by the accompanyingphotographic drawings, with colors therein being exhibited as nearlytrue as is reasonably possible with color illustrations of thischaracter.

FIG. 1 is a color of photograph of a branch of a ramet of the cultivarshowing typical leaves, (photo taken August of 1990, Chippewa NationalForest);

FIG. 2 is a color photograph showing this cultivar at 20 years of ageand in the center row of plantings, as compared with other plantings ofaspen trees of the same age, and illustrating the more vigorous growthhabit of this variety, (FIG. 2 being taken in the late autumn of 1988,Rosemount, Minn.).

FIG. 3 is a color photograph of the trunk of a mature ramet of thisvariety shown in a stand of aspen in the Chippewa National Forest, CassLake, Minn. (photo taken in December of 1988);

FIG. 4 is a composite photograph taken in August of 1990, and showing anindividual ramet of the mature clone as photographed in the ChippewaNational Forest, Cass Lake, Minn., and illustrating the mature shape andform when grown in such forest location;

FIG. 5 shows a cross-section of a mature ramet of the cultivar,illustrating the sound wood at 76 years of age;

FIG. 6 is a color photograph (taken in July of 1989) of a 24 year oldramet of the variety growing in a planting of the variety at theRosemount Agricultural Experiment Station, Rosemount, Minn. andillustrating the callus which inhibits the extension of Hypoxyloncanker;

FIG. 7 is a mature ramet of the variety growing in a planting at theChippewa National Forest, Cass Lake, Minn. and also illustrating thecallus growth which has prevented further expansion of the canker (phototaken in June of 1990); and

FIG. 8 is a photograph of plantlets of the variety generated by means oftissue culture (photo taken in June of 1990).

DESCRIPTION OF THE VARIETY

The following traits or characteristics have been repeatedly observedand are believed to be characteristics of this cultivar which, incombination, distinguish this variety of aspen tree as a new anddistinct cultivar.

No references to color are included in this description since all partsof the tree apparatus to be indistinguishable in color from the speciesand are typical thereof.

The ploidy of this Populus tremuloides has been determined to be anormal diploid, by means of chromosome counts and DNA measurements usingflow cytometry.

At the two above-noted plantations at the Rosemount AgriculturalExperiment Station and the Oconto seed orchard, the tree of this varietywas grown from asexually reproduced sprouts as clonal trees. The treeswere grown at a spacing of 3 meters×3 meters. The height and diametergrowth of the trees of this variety averaged 20% greater than any of theother Populus tremuloides selections grown at such plantations, over aperiod of 27 years at the Rosemount Plantation and for 22 years at theOconto Plantation.

The tree of this variety was selected because of its superior growthrate and aesthetic appearance. This selection develops uniformly spaced,strong lateral or scaffold branches, which arise from the trunk atuniform angles, while the central leader is distinctly straighter, ascompared to over 600 other clones or varieties of aspen trees. Thefaster growth rate and large vigorous canopy of this tree results inshading of the lower branches and thus the shedding or natural pruningof such lower branches. The result is a straight stem or trunk and avigorous crown, which makes the tree of this variety a dominant treewhen competing against other clones or varieties of aspen trees of itsown age. The vigorous growth rate of the tree of this variety also makesthe tree a good competitor in forest sites, as well as gardens andparks. Root suckering of the three of this variety is particularlyvigorous as compared to other varieties or selections of aspen trees.

THE TREE

The variety has a form typical of aspen trees, but with a more vigorousand rapid growth than is typical of the species and other varieties ofaspen trees.

In other respects, the variety is also typical of the species. The barkcolor; tree shape and habit; the leaf size, shape and color; buds; andflowers are not distinctive and are typical of the species, when grownin the same areas.

The parent ramet of the variety had a diameter at breast height of 55.5cm and was 33.84 m tall at 76 years of age.

Bark: The bark is smooth, gray-green on young trees, later becomingdeeply fissured and gray to black on the lower portions of the trunk ofolder trees, as illustrated in FIGS. 2, 3 and 4. The bark color is notdistinct from other members of species.

Leaf: Light green and glabrous above, duller green and glabrous belowduring growing season, and turning yellow in autumn.

Shape: Nearly orbicular, truncate or rarely subcordate at the base;short acuminate apex; evenly crenate-serrulate margin.

Aspect: Entirely glabrous when mature.

Size: Length from 2.5 cm to 5.0 cm; width from 2.3 cm to 5.00 cm.

Petioles: Length from 2.5 cm to 5.3 cm, approximately equal in length tothe leaf blades, slender and laterally flattened.

FLOWER

The variety produces deep red staminate flowers only, in catkins 3-6 cmlong, and pollen from the variety has provided vigorous progeny incrosses made over the past five years.

The tree of this variety has male flowers only, and staminate catkinsappear in advance of the leaves and are 3 to 6 cm long and less than 1cm thick. Stamens number 6 to 12 and deep red pollen sacs are born on adisk that is subtended by a 3 to 5 cleft, silky-hairy bract. Pollen fromthis tree has been used in a number of crosses, resulting in seedlingprogeny which are being evaluated in a genetic study on Populustremuloides.

FRUIT

None, since the variety produces pollen only.

BUDS

Bud scales are smooth, shiny, and deep brown in color. Terminal buds areconical, sharp pointed, slightly resinous, covered by 6-7 imbricatedscales.

HARDINESS

The cultivar is hardy and resistant to winter die-back.

DISEASE AND INSECT RESISTANCE

The cultivar has good resistance to Hypoxylon mammatum. Calluses usuallyform at the margins of the cankers and slow or completely stop cankergrowths, as illustrated in FIGS. 6 and 7. All known varieties of aspentrees are very susceptible to infection by such fungus. However, thisvariety exhibits more resistance to Hypoxylon mammatum by developing thenoted calluses which slow or completely stop the canker growths. In a20-year test at Rosemount, Minn. the other 574 aspen in the experimentaveraged 2.9 cankers per tree. Members of this clone averaged 1.8cankers per tree and callus tissue on the margins of these cankersslowed canker growth.

In the juvenile stage, the cultivar is susceptible to powdery mildewcaused by Uncinula salicis and shoot blight caused by Venturiamacularis.

A cutting of the parent ramet at 76 years of age produced a diameter of55.5 cm at breast height, and there was little white trunk decay causedby the fungus Phellinus tremulae, as noted in FIG. 5.

Insect Resistance

The cultivar has good resistance to wounding insects by rapidly formingcallus tissue to seal the wound.

WOOD

Wood produced by the tree of this variety is of high quality andgenerally free of stain and decay, as compared to wood from other knownvarieties of aspen trees. The rapid growth rate and natural pruninghabit with respect to the lower branches produces wood which isgenerally free of knots, as compared to other known varieties of aspentrees. The ability of this tree to form callus tissue in response towounds limits the amount of stain and decay fungi that enter the tree.The result is wood which is generally clean and of high quality,compared to the wood from most other known varieties of aspen trees. Itis therefore believed that the wood from trees of this variety will bein demand for pulp and fiber use.

We claim:
 1. A new and distinct variety of aspen tree, substantially asshown and described herein, characterized particularly as to novelty byrapid growth, good form, aesthetic appearance, and resistance toHypoxylon canker.